1. Field of the Invention
The invention relates to a liquid crystal alignment agent, a liquid crystal alignment film, and a liquid crystal display device. More particularly, the invention relates to a liquid crystal alignment agent that can be made into a liquid crystal display device having good uniformity of pretilt angle after ultraviolet irradiation, a liquid crystal alignment film formed thereby, and a liquid crystal display device having the liquid crystal alignment film.
2. Description of Related Art
So far, known liquid crystal display device include, for instance, various liquid crystal display devices of twisted nematic (TN)-type, super twisted nematic (STN)-type, vertical alignment (VA)-type, in plane switching (IPS)-type, fringe field switching (FFS)-type, or optically compensated bend (OCB)-type that use liquid crystal molecules having positive dielectric anisotropy. Moreover, to perform alignment control of each liquid crystal molecule, a liquid crystal alignment film (patent literatures 1-4) formed by an organic film is mainly used.
Due to the high-speed response of the liquid crystal molecules of, for instance, the TN-type or STN-type liquid crystal alignment film and the certainty of the inclination direction of, for instance, the VA-type liquid crystal alignment film when liquid crystals are driven, each of the TN-type or STN-type and the VA-type liquid crystal alignment film needs to have pretilt angle characteristics. A method of providing pretilt angle characteristics is, in the former cases, generally a rubbing method, and in the later case, is generally a rubbing method or, for instance, a method in which a protrusion is disposed on a substrate surface. In particular, dust or static electricity generated in the steps of the rubbing method sometimes causes the issue of poor display or a damaged circuit. Moreover, the method in which a protrusion is disposed on a substrate surface sometimes damages, for instance, the brightness of the obtained liquid crystal display device. The methods therefore all have drawbacks.
As a result, regarding a method of providing pretilt angle that is used to replace the methods above, a so-called photoalignment method (patent literature 5) in which ultraviolet is irradiated on a photosensitive thin film from a direction inclined relative to the normal of the film is provided. However, the liquid crystal display device made by the liquid crystal alignment agent recited in patent literature 5 still has the issue of poor uniformity of pretilt angle after ultraviolet irradiation, and is therefore not suitable for use.
Moreover, regarding a method in which liquid crystal alignment is provided to a coating film formed by a liquid crystal alignment agent containing, for instance, polyamic acid, in recent years, a technique using a photoalignment method such as photoisomerization, photodimerization, or photolysis has been proposed to replace the rubbing method. The photoalignment method is as follows: anisotropy is provided to a film by irradiating a radiation-sensitive organic thin film formed on a substrate with polarized or unpolarized radiation so as to control the alignment of liquid crystal molecules. According to the method, in comparison to the rubbing method above, the generation of dust or static electricity in the steps can be suppressed, and therefore the generation of poor display caused by, for instance, dust, or reduction of yield can be suppressed. Moreover, the photoalignment technique also has the advantage of, for instance, uniformly providing liquid crystal alignment to the organic thin film formed on the substrate.
Specifically, the technical literature of the photoalignment method is as shown in patent literature 6. Patent literature 6 provides a repeating unit having conjugated enone and a liquid crystal alignment agent having an imide structure. However, the liquid crystal display device made by the liquid crystal alignment agent recited in patent literature 6 also has the issue of poor uniformity of pretilt angle after ultraviolet irradiation.